1. Field of the Invention
The present invention relates to a light emitting structure and a light emitting device, which allow light to be extracted to the outside with efficiency to attain an improved luminance.
2. Description of the Related Art
Displays such as an electroluminescent (EL) display and a field emission display (FED) have been attracting attention as a flat panel display (FPD) using fine particles and a thin film both having a light-emitting function. The features of the EL display reside in that the EL display is self-luminous and is a perfect solid type, which is conducive to an excellent environmental tolerance.
Examples of the EL device include: an inorganic EL using electroluminescence produced by an inorganic material; and an organic EL using current injection luminescence produced by an organic material. In general, light generated in those EL devices is all reflected unless being made incident on an interface at an angle smaller than a critical angle, which makes it difficult to extract all the light to the outside. The light-extracting efficiency is typically around 20%, though depending on the material constituting the device. Examples of a method to improve the light-extracting efficiency include: a method disclosed in Japanese Patent Application Laid-Open No. 2002-278477, in which a low-refractive layer is provided between a transparent electrode layer and a transparent substrate; and a method disclosed in Japanese Patent Application Laid-Open No. 2004-296438, in which a transparent electrode is provided with an irregular surface.
According to the above-mentioned methods, however, it is necessary to provide the low-refractive layer or a light scattering layer in addition to the constituent elements such as the substrate and a light emitting layer, which makes the structure of the device and also a manufacturing process therefor complicated, leading to a difficulty in quality control of the light emitting device and to an increase in cost of manufacturing.
It is also possible to use a monocrystalline thin film for a light emitting part (thin-film type) (see FIG. 9A). In the case of using the monocrystalline thin film, excellent quantum efficiency can be assumed in general, whereas the light-extracting efficiency is limited to about 20%. Also, it is difficult to produce monocrystal in a large size, leading to an increase in cost.
Meanwhile, according to a conventional structure in which microcrystal or fine particles is used for a light emitting part (light emitting particle scattering type) (see FIG. 9B), the light-extracting efficiency is increased due to a scattering of light caused at the interface. At the same time, however, the surface area is increased, leading to an increase in the number of nonluminous centers, thereby decreasing the quantum efficiency. The quantum efficiency may be increased by adopting microcrystal in a relatively large size (0.1 μm or more), while adversely making the surface rough in general, which becomes an impediment in making the device thinner. In FIGS. 9A and 9B, reference numeral 91 denotes an electrode; 92, an insulating layer; 93, a dielectric film; 94, a light emitting layer; 95, a transparent electrode; and 96, a glass substrate. The above-mentioned problems can be solved by the following structure and manufacturing method according to the present invention.